Synthesis of nanocrystals with defined sizes [1-3] and shapes [4-10] has advanced dramatically in recent years. High temperature approaches (roughly 250-350° C.) in organic solvents, either through organometallic schemes [1, 5, 11] or greener approaches [12-14] have played a key role and often been regarded as the mainstream synthetic chemistry in the field. The organometallic and greener approaches are commonly referred to “pyrolysis” or “thermolysis” of the precursors under high temperatures. Emphasis on synthetic chemistry of nanocrystals is currently moving into nano-objects with complex structures and compositions [15]. Particularly, formation of three-dimensional colloidal nanocrystals has drawn a great deal of attention to researchers.
Development of the mainstream synthetic chemistry for zero dimensional (0D) and one dimensional (1D) structures have been greatly benefited by the mechanistic studies on the growth of nanostructures. However, all these studies are mostly limited to growth kinetics [15], and no systematic information on chemical reaction mechanisms of typical synthetic processes is reported. Such chemical reaction mechanisms may become critical in developing general strategies for synthesis of three dimensional (3D) complex nanostructures, provided the expected complex structures change in the reaction system. Synthetic schemes involved in the formation of oxide nanocrystals is a good starting point for such studies, since the synthetic process is extremely simple, like the thermal treatment of a fatty acid salt in a hydrocarbon solvent [14]. In addition, the chemical reaction information also provides valuable references for understanding reactions involved in the formation of other types of inorganic nanocrystals in the mainstream synthetic chemistry. At present, metal fatty acid salts are the most common precursors used in synthesis of all types of inorganic nanocrystals in non-aqueous media under elevated temperatures. Presumably, thermal stability of these precursors under synthetic conditions is of great importance for designing and understanding any of these synthetic schemes.
The importance of the above mentioned crystal growth mechanism can be illustrated with an example of synthetic chemistry for 1D nanowires, i.e., solution-liquid-solid approaches [4] and 1D oriented attachments [6, 7, 10, 16-18]. For the 1D oriented attachments, the electric dipole moment of the nanocrystals plays a determining role. The dipole moment of a given type of crystal has a given set of possible orientations as indicated by a recent impressive study in Murray's group [10], which generates nanorods/wires and related structures with a given set of orientations.
Nanocrystals with complex 3D structures have widespread applications in solar cells, catalysis, sensing, and other surface/shape related characterizations. For instance, CdTe and other semiconductor tetrapods [21, 22] are ideal structures for fabrication of high performance solar cells [23]. Such tetrapods, however, are typically formed by a traditional path, atom by atom growth from nuclei, and the intrinsic crystal structures play a key role. However, it is not clear how to extend the synthetic methods to different structures. Several reports indicate that nanodots and nanorods can self-assemble into different complex shaped particles [24-26]. Such complex structures, however, are often quite large, fragile, and/or polycrystalline. Other reports indicate possibilities for formation of complex nanostructures through 3D attachments, however, these reports are typically brief, missing clear mechanistic evidences, or structurally not well characterized [27]. Thus, a general pathway to form 3D oriented attachments has not been achieved yet.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.